From 1970s IC industry has made dramatic progress due to the advancement of photo- lithography technology. At the beginning of the 21st century, photo- lithography technologies are still acting the major role of manufacturing the leading edge devices. The requirement for the higher resolution is pushing up the NA of the projection lens. As the result, DOF becomes shallower and the focus budget becomes tight. Close study for the focus error impact to CD variation becomes more and more important. A method to predict focus error induced CD variations resulting from dynamic wafer scanning has been developed. CD variations across an exposure image field are calculated from a CD lookup table that relates a CD value to the monitored focus error components.